The present invention relates to a method and an apparatus for estimating a mass of a vehicle as well as to a method and an apparatus for estimating a gradient employing the method.
In apparatuses for alarming air-pressure of indirect type which are based on relative comparison of wheel speeds or the like, it is known that physical quantities (such as dynamic load radii) which indirectly represent air-pressure are largely affected by ways of driving, road conditions such as gradients or road surface friction coefficients μ, or load conditions of the vehicle. For eliminating such influences, it has been conventionally performed that data containing therein a large number of errors are identified from among wheel speeds or pieces of information of sensors provided in the vehicle, and that corresponding pieces of data are corrected or eliminated.
However, as for uphill/downhill driving and changes in weight owing to the number of passengers or the amount of luggage or to the presence/absence of a traction vehicle, such data cannot be identified and accordingly corrected or eliminated although such factors largely affect the above-described indirect physical quantities such as dynamic load radii.
For instance, Japanese Unexamined Patent Publication No. 242862/1997 describes an apparatus A for estimating road surface gradients. This publication utilizes the fact that the relationship of vehicle driving torque=flat surface running resistance torque+acceleration resistance torque+gradient resistance torque is satisfied since the vehicle driving torque balances with the sum of the flat surface running resistance torque, the acceleration resistance torque and the gradient resistance torque. The gradient resistance torque is then calculated upon obtaining the vehicle driving torque, the flat surface running resistance torque and the acceleration resistance torque so as to obtain a road surface gradient sin θ from the following equation.Tθ=W×g×sin θ×RtHere,                Tθ: Gradient resistance torque        W: Vehicle weight        g: Gravitational acceleration        Rt: Load radius of tire.        
In the above apparatus A, the flat surface running torque and the acceleration resistance torque are calculated by using preliminarily obtained data for calculating a gradient resistance torque upon performing deductions so as to calculate a gradient. However, since both of the flat surface running torque and the acceleration resistance torque are amounts which are dependent on a mass of a vehicle or an air resistance coefficient thereof, it is possible that errors become larger when the vehicle is towing a trailer, when many pieces of luggage or passengers are riding thereon or when a roof carrier is mounted onto the roof of the vehicle.
In contrast thereto, it is possible to employ, instead of driving torque, slip rates which can be obtained from wheel speeds of the front and rear wheels and to detect gradients by using data in which the vehicle acceleration is proximate to zero for the purpose of eliminating influences of acceleration resistance torque.
When employing this idea, gradients are detected upon performing regression and obtaining slip rates when the acceleration is zero for the purpose of eliminating influences of acceleration resistance torque; however, it is impossible to detect gradients at high accuracy since it is difficult to take influences of wear of tires through running, of changes in the entire weight caused through luggage or the number of passengers, and of changes in front and rear distribution of load into consideration.